Erosion due to canals Erosion due to natural processes Subsidence Draining and Filling

Salt Water Intrusion Sea level rise Lack of sediment deposition

Erosion due to canals
Written by Leigh Casadaban

As a threat to interior wetlands, canals are among the many man-made engineering projects that affect nature on a large scale.  Canals were created in Louisiana’s wetlands to serve two main purposes: allow industrial traffic to the port city in the form of navigation canals, and transfer oil and gas to locations around the nation in the form of pipeline canals.  Starting in the 1940s, a surge of engineered navigation canals turned natural delta tributaries between New Orleans and the Gulf of Mexico into stricter paths, as well as created random barriers through wetlands, inhibiting exchange of water, sediment, and nutrients.  Ten major navigation canals now exist for ships and barges that require deep, unobstructed waterways for industrial transportation.  The wetland area, which once relied on constant sedimentation from river flooding, began to dry and sink below the water level, causing them to disappear.  Also, pipeline canals extend for more than 38,000 kilometers, cutting through ecosystems of Louisiana’s coast.  Aside from wave induced erosion, these deep canals introduce saltwater to the wetlands and contaminate marshes and freshwater ecosystems.  While some brackish ecosystems develop, saltwater intrusion does erode vegetation and kills wildlife habitat in more sensitive freshwater ecosystems, causing erosion from within.  This internal erosion has affected the wetlands’ ability to absorb tropical storms before they reach inland cities like New Orleans.  As a result, the city has been more susceptible to storm damage. Industry representatives admit responsibility for 10 to 20 percent of wetland loss, but others have suggested that industry canals cause anywhere from 50 to 80 percent of erosion. 

(lacoast.gov and USGS press  release: "Without Restoration, Coastal Land Loss to Continue").

Weathering and erosion are natural processes.  Weathering causes the breakdown of parent material into soil, and wears away at mountains and cliffs.  Erosion is when existing sediment is transported by wind and water to new locations.  When nature is left to her own devices, the rate of erosion equals the rate of soil formation, so the cycle is always in a state of equilibrium (“Natural Erosion,” 2006).  It is when human activities catalyze the rate of erosion without doing anything to supplement the rate of soil formation that ecosystems run into trouble.  The Louisiana wetlands are especially vulnerable because the amount of water that flows through them on a daily basis washes away sediment, which is no longer replaced by river overflow since human interaction has put extreme constraints on the river.  The USGS estimates that between 1932 and 2000 Louisiana lost 1,900 square miles of coastal marshland (2006).

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Louisiana’s coast has seen natural subsidence since the delta’s formation. Due to man made engineering river diversions, however, the land has not been able to actively replenish its marshes. Until recently, causes of subsidence have been defined as strictly natural, not industry related.  However, petroleum geologist Bob Morton, of the U.S.  Geological Survey, has recently found evidence for industry induced subsidence. Millions of barrels of oil and trillions of cubic feet of natural gas extracted from the ground, causing regional depressurization, a drop in subsurface pressure. Regional depressurization causes underground faults to slip and surface land above to slump.  This affects the course of waterways and leads to land loss by sinking wetlands below water levels.  Based on this evidence, and the fact that petroleum wells are running dry in Louisiana’s wetlands, subsidence rates are expected to decrease in the future.  However, other potential energy resources, including geopressured-geothermal fluids, could lead to further subsidence (Robert A. Morton, 2006).

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Draining and filling are the most direct ways in which humans can impact the wetlands.  During the early part of the nineteenth century Louisiana was growing at a very rapid rate, and the local government made development a much higher priority than the protection of the environment.  During this time many of the wetland ecosystems were drained and filled in order to provide more stable soil for agricultural areas and expansion of Louisiana communities.  Now, after the devastation of Katrina has driven everyone away from the Gulf Coast, officials are trying every method to bring them back.  In Mississippi the Army Corps of Engineers actually plans to relax zoning laws so that developers can easily build on five-acre parcels of land in wetland ecosystems (Stuckey, Mike,  2006).  This could be applied next to the lower regions of Louisiana as an incentive to bring people and businesses back to the areas of greatest destruction.   This is not only unsafe for the people building there, since wetland soil is very unstable, but it also increases the rate at which Louisiana is losing it’s wetlands.

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Salt water intrusion is a major cause of wetland loss and is increased by canal systems.  Currently, there are 10 major navigation canals and countless smaller ones winding intricately through the wetlands of southeast Louisiana.  These canals are used for transportation for oil companies that drill in the wetlands, and for the agricultural areas in the region.  However, they connect the inland, freshwater wetlands with the Gulf of Mexico.  This increases the salinity of the freshwater areas causing vegetation deterioration and land loss.  Also, because of the flow of traffic through the canals and the instability of the surrounding soil, the wetlands are very easily eroded.  Consequently, the brackish water penetrates even further into the wetlands.  This increase in salinity is toxic to many of the plant species that grow there.  Much of the time the community cannot handle the sudden increase in salinity, and the plants die.  The fewer plants there are in the wetlands the more unstable the soil is because the plants' root systems hold much of the substrate together.  The loose soil is then more easily eroded, which connects the vicious circle of erosion and plant loss.

Due to changes in the global environment, it is predicted that the sea level will increase over the next few years.  The increase can be accredited to melting ice caps and thermal expansion. In the next century, it is expected to be approximately one meter higher than the current level.  Since Louisiana is a very low lying state, the rise will greatly affect the geography of the coast. 

The productivity of wetlands is limited by the depth in which they grow.  If the water is deeper, there is less oxygen available and many biogeochemical functions in the soil are disturbed.  At plants’ roots, O₂ acts as an electron donor in reactions with vital elements such as nitrogen and sulfur.  When the sea rises, O₂ will not be present at the plants' roots because of the distance from the surface where oxygen diffuses into the water.  Additionally, global warming will increase the temperature of the shallow water in the wetlands.  Liquids of higher temperatures are less capable to holding dissolved gases decreasing the amount of oxygen even more (Neubauer, 2006).

The warmer temperature will also increase the rate of evapotranspiration, the effects of both evaporation and transpiration, which in turn will decrease the ratio of water to salt and increase the salinity of the water.  Plants of freshwater wetlands cannot handle the change in their environment and will deteriorate.  Without vegetation, the rate of erosion will increase, leading to an escalation in the disappearance of wetlands (Morris, 2006).

In the Mississippi River basin, erosion, weathering, and deposition of sediments occur in a dynamic equilibrium.  Generally, the rate of erosion equals the rate of sediment deposition, so the cycle is in a state of equilibrium --- the wetlands are neither being eroded nor growing (“Natural Erosion”).  Because the Mississippi River delta is such a complex and changing system, some areas will receive a net input or output of sediments, and therefore either grow or be eroded.  This accounts for the naturally changing geography of the Mississippi River basin.

However, in the 1930s, the preexisting sediment balance of the Mississippi River Delta was upset by artificial control of the Mississippi River.  Upstream, dams lower the Mississippi’s speed and energy, allowing fine-grained sediments to settle out.  Additionally, diversions of the Mississippi have caused some areas that were previously inundated with sediment and nutrient laden water to become isolated, cutting off their supply.  This artificially decreased input of sediments has caused a dramatic increase in the areas experiencing net erosion, and has accelerated the rate of degradation.

The decrease in the quantity of sediments entering the Mississippi River Delta system has also notably affected the coastal barrier islands.  The decreased amount of sediment has accelerated the rate of natural erosion of Louisiana’s islands, which play a major role in protecting the coast from erosion due to wave energy in events such as a hurricane.